Polymerization of dl-alanine carboanhydride and orientation of polymer



United States Patent POLYMERIZATION OF DL-ALANINE CARBOAN- I-IYDRIDE ANDORIENTATION OF POLYMER Ralph E. Miegel, Elsmere, Del., assignor to E. I.du Pont de Nemours and Company, Wilmington, Del, 1: corporation ofDelaware No Drawing. Application November 24, 1950, Serial No. 197,513

2 Claims. (Cl. 260-775) This invention relates to new and usefulalpha-amino acid polymers.

The N-carboanhydrides of alpha-amino acids have recently beendemonstrated to offer a possible route to high molecular weightalpha-amino acid polymers, for instance, as reported by Woodward andSchramm, J. Am. Chem. Soc., 69, 1551 (1947) and in the copendingapplication of MacDonald, Serial No. 766,457, filed August 5, 1947, nowPatent Number 2,572,844. However, both of these reports deal solely withcopolyamides. The teachings of this art, whether implicit or explicit,is to the effect that only copolyamides containing units from at leasttwo different alpha-amino acids are capable of forming organic solventsoluble polymers of sufliciently high molecular weight to be capable ofbeing formed into useful films and fibers.

In the copending application of Cleaver and Schreiber, Serial No.71,756, filed January 19, 1949, now abandoned, it is pointed out that anorganic solvent soluble filmand fiber-forming alpha-amino acidhomopolymer can be prepared from synthetic optically inactivealphaaminoisocaproic acid. Although this homopolyamide is capable offorming tough, strong, cold-drawable films and fibers, such products donot exhibit sufiicient water sensitivity to. make them as readilydyeable or as easily handleable by normally used aqueous processingtreatments as is desired. These latter two properties are of the utmostimportance in the preparation of fibers which are to have outstandingutility in the present wide field of synthetic fabrics.

This invention has as an object the preparation of a homopolyamide whichis tough, strong, and cold-draw-' able to films and fibers and which isreadily dyeable and processable by normally used aqueous textileprocessing treatments. Other objects will appear hereinafter.

These objects are accomplished by the present invention of oriented,high molecluar weight polyamides consisting solely of units fromDL-alanine which afford strong, touch, organic solvent soluble films andfibers capable of being readily dyed and processed by normally usedaqueous treatments: The invention also includes the preparation of thepolymer by polymerizing DL-alanine N-carboanhydride [prepared byreacting DL-alanine 'or its salts with phosgene under anhydrousconditions as disclosed in the copending applications of MacDonaldSerial No. 778,458, filed October 7, 1947, now abandoned, and Serial No.83,299, filed March 24, 1949, now Patent Number 2,662,084, preferably inthe presence of an ether type solvent as disclosed in the copendingapplication of Prichard Serial No. 52,971, filed October 21, 1948, nowabandoned] by heating the N-carboanhydride at temperatures in the range55 to 115 C., preferably 60 to 85 C. for at least 20 hours in an inertliquid solvent for the monomer which is a non-solvent for the polymer,preferably in benzene solution, at a concentration not exceed ing 14%and preferably in the range 0.5 to 10.0% and continuing thepolymerization until the DL-alanine polyamide formed exhibits aninherent viscosity of at least 0.3, isolating the high molecular weightpolyamide so formed and finally orienting it by drawing at least Sincethe films and fibers formed therefrom exhibit greater strength, it ispreferred to continue thepolymerization until such time as theDL-alanine polyamide exhibits an inherent viscosity of 0.6.

DL-alanine polyamide of relatively high molecular Weight and ofpartially satisfactory properties in filmand fiber-form can be preparedby polymerizing DL-alanine N-carboanhydride in solution in an inertsolvent for the monomer which is a non-solvent for the polymer andliquid at the polymerization temperature, e. g., a halogenatedhydrocarbon or an aromatic hydrocarbon, under similar conditions.However, DL-alanine poly- F amide of the quality desired for outstandingfilms and fibers has thus far been most readily prepared bypolymerization in benzene solution. Such a polymerization procedure is,therefore, accordingly preferred. Although polymeriztions may be carriedout in halogenated or aromatic liquid hydrocarbon reaction media athigher concentrations, no successful polymerizations to the orientablehigh molecular weight poylmers of this invention have been obtained atsolids concentrations higher than about 14%. benzene as a polymerizationmedium at a solids concentration not exceeding about 14%, is, therefore,accordingly preferred.

Because of the greater ease of preparation of the DL-alanine polyamidein the requisite ranges of molecular weight as evidenced by the inherentviscosity, in the minimum amount of time, it is preferred to use highlypurified DL-alanine N-carboanhydride as the starting material. Manymethods for purifying the DL-alanine N-carboanhydride suggest themselvesto those skilled in the art, such as recrystallization from limitedquantities of a solvent, preferably taking advantage of the usualsolubility/temperature relationship, sublimation, preferably under veryhigh vacuum, and the like. A particularly convenient method of obtainingthe DL-alanine N-carboanhydride in the ranges of purity necessary toenable the preparation of the preferred high molecular weight DL-alaninepolyamide is to recrystallize the N-carboanhydride by a precipitation/crystallization technique from anhydrous diethyl ether/ petroleum ethermixtures.

To carry out the last step, i. e., the orienting, described previouslyin the preparation of the oriented DL-alanine polyamides of thisinvention, it has been found necessary that the DL-alanine polyamide befirst swollen by contact with a solvent or swelling agent for thepolyamide. In

the preparation of the oriented DL-alanine polyamide of this inventionin film form, this normally means that the films once obtained (that is,after the casting solvent has been removed) must be contacted with asolvent or a swelling agent and then oriented. The lower alkanols (i.e., saturated, aliphatic monoalcohols of from one to four carbons which,other than the single hydroxyl group, are solely hydrocarbon in nature)have been found to be particularly convenient for use in this step andalso capable of producing particularly outstanding, highly oriented andstrong films.

For the preparation of the oriented DL-alanine polyamides of thisinvention in fiber form, no separate swelling step is usually necessary,particularly in the normally used wet-spinning procedures. In thisprocess, the fibers are formed by pressure ejecting a solution of thepolyamide through very small orifices into a coagulating bath. At thisstage the fibers are partially solvated by residual traces of theoriginal solvent, and also partially swollen by the coagulating bath,which in most instances will be a lower alkanol or acetone-based bath.The fibers in this condition can be readily oriented by ;im-

A polymerization procedure using grams per 100 cc. of solution.

preferably at least 300% of the undrawn length by drawing while hot, forexample while immersed in a silicone bath at 225-275 C. For the normallyobtained high molecular weight polyalanine, i. e., rangingin molecularWeight from 15,000 to 45,000, it is preferred to carry out the drawingso that an overall draw of from 2X to 3X, i. e., to 200 to 300% of theundrawn length is obtained in both types of drawn fibers as well as thedrawn filmsy The following examples in which the parts given are byweight are presented to further illustrate this invention. As usedtherein, inherent viscosity mh.) is defined by the following equation:

wherein In is the natural logarithm,

11 solution 1 solvent 1 being viscosity, C is the concentration of thesolute in Unless otherwise noted all inherent viscosity results areobtained with the polyamide involved dissolved in m-cresol at aconcentration of 0.5 gram of polymer per 100 cc. of solution at 25 C.Intrinsic viscosity, [1;], as used in these examples is defined by thefollowing equation:

wherein In, C, and 17ml. are as defined above. In all instances theintrinsic viscosity figures are determined frorna series of resultsobtained with the polyarnide involved dissolved in m-cresol at variousdecreasing concentrations from 0.5 gram per 100 cc. of solution andbelow at 25 C.

EXAMPLE I to room temperature and filtered under anhydrous con ditionsto remove the insoluble material parts of DL- alanine hydrochloriderepresenting .about of the starting material). The filtrate is thenconcentrated by distillation under reduced pressure at to 40 C. to about10% of its original volume. The oily concentrate thus obtained is washedwith about parts of petroleum ether which is then decanted and theresidue taken up in about 280 parts of anhydrous diethyl ether.Approximately 5 parts of decolorizing charcoal are added to theresulting solution and the mixture allowed to stand at room temperaturefor about ,20 minutes with occasional shaking. The decolorizing charcoalis, removed from the mixture by filtration and the filtrate diluted withabout parts of petroleum ether and the mixture then cooled in anice-water bath. DL-alanine N-carboanhydride begins to precipitate out ina relatively short time. When it appears that no further precipitationis occurring, approximately 250 parts of petroleum ether are then addedand the resulting mixture cooled for about an ad ditional hour in theice-water bath and the crystalline product then removed by filtration.There is thus obtained 37 parts of DL-alanine N-carboanhydridecorresponding to a conversion of and a yield of 91%,

taking into account the 5 parts of DL-alanine hydrochloride removed fromthe reaction zone as described above.

EXAMPLE II A 27.7 part sample of DL-alanine N-carbonhydride prepared inessentially the same fashion described previously in Example I isdissolved in parts of anhydrous diethyl other at 3035 C. No visiblesolid is noticed in the resulting solution although the solution isfiltered as a precautionary measure. To the filtrate is addedapproximately 30 parts of petroleum ether and the cloudy solution thusobtained is cooled for about 20 to 30 minutes in an ice-Water bath. Atthe end of. this time there is no further indication of the formation ofthe solid which had begun to precipitate out when the solution was firstcooled to 0 C. An additional 330 parts of petroleum ether is added andthe mixture again cooled 'in an ice-water bath for about 20 to 30minutes at the end of which time no further separation of product isnoticed. The white solid crystalline material is removed by suctionfiltration and dried on the filter under anhydrous conditions. There isthus obtained 17.85 parts of purified DL-alanine N-carboanhydride. Asample of this material in a thin-walled glass capillary, when immersedin a melting point bath at 42 C., melts sharply at 44.5-45.5 C. as thetemperature of the bath is slowly increased. Another sample of thisDL-alanine N-carboanhydride, when immersed as described above in amelting point bath at 45 C., melts almost immediately at 45.0-45.5" C.as the temperature of the bath is slowly increased.

A portion of the above, once-recrystallized DL-alanine N-carboanhydride,is recrystallized again under the same general conditions except thatrelatively more anhydrous diethyl ether and appreciably less petroleumether are used. Both these concentration changes, of course, tend tomake the recovery of the DL-alanine N-carboanhydride less efficient andthe quality of the product ultimately obtained thereby higher. Theultraviolet absorption spectrum is measured for a 1% solution of thistwicerecrystallized DL-alanine N carboanhydride in purified anhydrous,dioxane. This spectrum indicates that purified D -alanineN-carbo-anhydried exhibits a specific absorption coefiicient of maximumvalue in the neighborhood of 2550 A. falling ofl rapidly until a valueof less than 0.0015 is reached in the neighborhood of 2700 A. remainingless than 0.0015 throughout the range from 2700 A. up to and including3200 A. with the general trend being that of a further slow decrease.

As used here and elsewhere in this specification, the specificextinction, k, is defined by Brode Chemical Spectroscopy, secondedition, Wiley 1943), page 190. The specific values of k given hereinwere obtained using a 'Beckman spectrophotometer (page 174, ibid.) andcalculated in concentration units of grams per liter and distance unitsof centimeters.

EXAMPLE III Preparation 0 high molecular weight DL-alanine homopolyamideof six days. The product which separates out during this time is removedfrom the polymerization mixture by filtration, vacuum dried, and takenup in formic acid. The resultantviscous solution is flowed in a thinfilm onto a glass plate and allowed to dry at room temperature. Afterremoval from the casting surface, there is obtained a clear, strong,tough film of a high molecular weight DL-alanine polyamide. Samples ofthis film exhibit inherent viscosities of 0.43 and 0.40 at 0.1 and 0.3%concentrations, respectively. The DL-alanine polyamide films so preparedexhibit no orientation when examined by standard X-ray procedures. Thisis particularly surprising since the alpha-amino acid polyamides thusfar known to us when cast into films by this same procedure exhibitrelatively high degrees of orientation.

Samples of a DL-alanine polyamide of inherent viscosity of 0.73 informic acid (prepared in essentially the same manner) are found to beinsoluble in saturated lithium bromide, saturated urea, and ammoniacalcopper oxide- (169 parts ammonia per liter and 30-35 parts Cu++ perliter (aqueous solutions even after standing for 21 hours at roomtemperature and for two and onehalf hours on a steam bath. Similarlysamples of the polyamide failed to dissolve in solutions prepared fromequal volumes of water and, respectively, saturated aqueous lithiumbromide and saturated aqueous urea solutions under the same conditionsof time and temperature. This polyamide is soluble in formic acid,m-cresol, and 88% aqueous phenol whereas, optically active alaninehomopolyamides, i. e., homopolyamides from D- or L- alanine areinsoluble in these solvents (see the copending application of MacDonaldSerial No. 108,327, filed August 2, 1949). Go and Tani, Bull. Chem. Soc.Japan, 14, 510 (1939) disclose the insolubility of low molecular weightpowdery L-alanine homopolyamide in all common protein solvents andGerman patent application 70,168 Ive/39c (PB34,279) discloses thatDL-alanine homopolyamide of medium molecular weight (of the order of5,000) is soluble in such solvents as aqueous lithium bromide,aminoniacal copper oxide, aqueous urea, etc.

A 0.88 part sample of a DL-alanine homopolyamide in film form[-mnn.=0.53 and 0.46 at 0.1 and 0.3% concentrations, respectively]prepared in essentially the same manner is suspended in 100 parts ofwater and heated at 90 C. for 1.5 hours and then dried at 65 C. The filminitially measures 44 mm. wide, 171 mm. long, and 6.7 mils thick. Thewater-extracted film, after drying, weighs 0.63 part, thus indicating aweight loss of approximately 29%. The film, while swollen, i. e., afterthe water treatment but before drying, measures approximately 52.4 mm.wide, 203.5 mm. long, and 7.6 mils thick. These swollen dimensionsindicate, therefore, approximate 19, 19, and 14% increases in thelength, width, and thickness of the film. On a volume basis, this meansthat the film on treatment with the 90 C. water swells to approximately1.7 times its original dry volume. Samples of the watertreated and driedfilm exhibit inherent viscosities of 0.54 and 0.51 at 0.1 and 0.3%concentrations, respectively.

Conversely, a 0.132 part sample of an L-alanine homopolyamide in filmform [1 ]=3.20 in dichloroacetic acid) is suspended in 200 parts ofwater and heated at 82 C. for one hour and then dried at 65 C. The filminitially measures 81 mm. long, 19 mm. wide, and 2.8 mils thick. Thewater-extracted film after drying weighs 0.132 part, thus indicating aweight loss of less than 1%. The film while swollen, i. e., after watertreatment but before drying, measures approximately 82 mm. long, 19.5mm. wide, and 3.2 mils thick. These swollen dimensions indicate,therefore, approximate 1.25%, 2.60%, and 14.30% increases in the length,width, and thickness of the film. On a volumebasis this means the filmon treatment with hot water swells to" only about 1.18 times itsoriginal dry volume. On the other hand, similarly, the medium molecularweight DL-alanine homopolyamide described in the previously identifiedGerman patent application is characterized by swelling ten times its dryvolume in water. These water swelling data serve to illustrate furtherthe surprising differences in kind existing between the various alaninepolyamides of the art and the oriented high molecular weight DL-alaninehomopolyamides of this invention.

Molecular weight determinations carried out on samples of the highmolecular Weight DL-alanine homopolyamides of this invention preparedessentially in the same manner as discussed immediately above indicatesuch polyamides to be high molecular weight. For instance, samples of aDbalanine homopolyamide ([n]=0.4l in formic acid) exhibit an apparentweight average molecular weight of 30,000 to 40,000 as determined bylight scattering measurements. Similar determinations carried out on aDL- alanine homopolyamide of [1 =0.75 indicate an apparent weightaverage molecular weight of 55,000 to 75,000. In both these instancesthe light scattering observations were obtained on solutions of thehomopolyamides involved dissolved in 98-100% formic acid.

Another similar polymerization carried out in reagent benzene at thereflux for five days with DL-alanine N- carboanhydride thricerecrystallized from ether-petroleum etherfurther varying only in thatthe concentration of the polymerization solution was appreciably lower(about 0.9% DL-alanine N-carboanhydride)yielded appreciably highermolecular weight DL-alanine homopolyamide. This homopolyamide exhibitedan [1 in formic acid of 0.90.

EXAMPLE IV A formic acid solution of a high molecular weight DL-alaninehomopolyamide ('I]inh.=0.34 at 0.1 and 0.3% concentrations), prepared ingeneral as described in Example III, is flowed in a thin film onto aglass plate, and the formic acid allowed to evaporate at roomtemperature. The film is removed from the casting surface and soaked fora short time in a 28 alcohol bath and then stretched to three times itsoriginal length, i. e., drawn 3:1. After being dried for two days atroom temperature, a sample of the drawn film exhibits a tenacity of12,100 lbs./sq. in. and an elongation of 4%. Samples of similar filmscast in a similar fashion exhibit appreciably lower tenacities prior toorientation.

Samples of the oriented DL-alanine .homopolyamides of this invention infilm or fiber form, prepared as described generally above, exhibit uponexamination an X-ray chain identity period of 6.60 to 7.00:0.05 A. Thischain identity period is characteristic of the natural beta-keratin typepolymers and is in direct contrast to the characteristic 5.10 to 5.40-*-0.05 A. X-ray chain identity period of natural alpha"-keratin typepolymers, such as, for example, the high molecular weight, opticallyinactive, synthetic polyamide from DL-alpha-aminoisocaproic acid(discussed in greater detail in the copending application of Cleaver andSchreiber Serial No. 71,756, filed January 19, 1949). The X-ray chainidentities and properties of these two characteristic groups ofsynthetic alpha-amino acid type polymers are discussed in greater detailin the copending application of Boynton Graham Serial No. 173,690, filedJuly 13, 1950, wherein the alpha and beta" type polymers are for clarityof ref-? erence discussed as type I and type II polyamides. The phrasechain identity period is defined at page 188 of Bunns ChemicalCrystallography, Oxford, 1945.

EXAMPLE V A film of a high molecular weight DL-alanine homo polyamide(mnh.=0.40 at 0.1% concentration and 0.43 at 0.3% concentration) cast asdescribed previously from formic acid solution is washed forapproximately 30 minutes in water at about C. and dried under vacuum at50 C. The resultant film is then dissolved in formic acid solution andrecast as before. Samples of this film exhibit an 71lnh.=0.47 at both0.1 and 0.3% concentrations. Strips of this recast film crack when bentthrough an angle of when the resulting bend is creased.

I After drying, the film measures 73:2 mm. in length.

The film showed no evidence of cracking or breaking when subjected tothe same bend and crease test through an angle of 180 previouslydescribed. X-ray diffraction patterns obtained on the drawn film clearlyshow that the film is highly oriented.

EXAMPLE VI A 0.352 part portion of a formic acid cast film of aDL-alanine homopolyamide (mnh.=0.57 at 0.1 and 0.3 concentrations),prepared in general as described pre viously in Example III, issuspended in 200 parts of water and the suspension heated on a steambath for 50 minutes at 90 C. The film is removed from the water anddried at 65 C. There is thus obtained a 0.270 part film (77% recovery)which exhibits flinh. of 0.73 at 0.1% concentration and 0.69 at 0.3%concentration. A 0.226 part portion of this film is again waterextracted as described previously. There is thus obtained a 0.220 partfilm, which upon a further water extraction in the manner previouslydescribed, remains constant in weight. This film is then dissolved informic acid solution and recast. A 0.218 part portion of this film isthen extracted in 200 parts of water for 1.5' hours on a steam bath at90 C. After drying, there is thus obtained a 0.188 part (86.3% recovery)film which exhibit 77inh.=0.73 at 0.1 and 0.3% concentrations. Thus,.water extraction improves the overall molecular weight or" theDL-alanine homopolyamides, presumably through'removal of lower molecularweight portions of the DL-alanine homopolyamides comprising the sample.Furthermore, theimprovement in overall molecular weight throughselective water extraction is made more efiicient in repetitive stagesif the homopolyamide sample involved is dissolved and recast betweenextractions.

EXAMPLE VII Fifty-three (53) parts of a high molecular weight DL-alanine homopolyamide ]=0.5 in cresol), prepared in general as describedpreviously in Example III, is dissolved in 212 parts of freshlydistilled, water-white m-cresol by stirring under nitrogen for two andone-half hours at room temperature and for two and one-half hours at 70C. The resulting solution is pressure filtered with nitrogen throughfelt and pressure spun at the rate of 2.4 ml. per minute through a -hole(0.025 inch hole diameter) platinum spinneret into an acetonecoagulating bath at C. The yarn travel in the coagulating bath is 36inches and the windup of the coagulated yarn on the Godet wheel is atthe rate of 27.9 feet per minute. The yarn is stretched 4.4 times in airat room temperature in being removed from the Godet wheel to a takeupbobbin at 123 feet per minute, washed in acetone at room temperature toremove residual m-cresol and then dried, sized and twisted 2.5 turns perinch. The following representative data are given for various samples ofthis yarn:

1 Incline plane, Where g. p. d.=grazns per denier.

The sticking point of the drawn yarn described above under entry III is245 C.

' to evaporateat room temperature.

Another sample of -the same spinning solution is spun as described aboveexcept that the yarn is stretched only 1.8 times in air at roomtemperature in being taken off from the Godet wheel. It is furtherstretched 3.0 times in a liquid heat transfer bath at 260 C. through a 4inch bath travel with the input rate of the yarn of 16 feet per minuteand the yarn takeup out of the bath being at the rate of 48 feet perminute. This yarn exhibits a denier of 108 and a tenacity and elongationas measured on the incline plane of 1.0/3 g. p. d./ 1%. Another sampleof yarn prepared from a further portion of the same spinning solutionunder similar conditions except that the yarn is stretched in air 3.5times at room temperature in being taken off the Godet wheel. This yarnis then further stretched 1.5 times in a liquid heat transfer bath at260 C. through a 4 inch bath travel with the input rate of the yarnbeing 32 feet per minute and the yarn takeup out of the bath being atthe rate of 48 feet per minute. This yarn exhibits a denier of 104 and atenacity/elongation of 1.2 g. p. d./2% as measured on the incline plane.X-ray examinations of various similar drawn yarn samples indicate thedrawn yarn to be oriented in the directionsparallel to the axis of thefilaments. Various samples of such yarns are dyed by selected acid,acetate, direct and vat dyes.

EXAMPLE VIII A solution of 2.0 parts of DL-alanine N-carboanhydride[prepared as described previously in Example I and purified by onerecrystallization as described previously in Example II] in 74.5 partsof purified anhydrous chloroform (B. P. 613 C.) is prepared and filteredto remove the small traces'of insoluble material. The resulting clearsolution is heated at the reflux under anhydrous conditions for 71hours. The polymer product which separates out during this time isremoved from the polymerization mixture by filtration, vacuum dried andtaken up in formic acid. The resultant viscous solution is flowed in athin film onto a glass plate and the formic acid allowed to evaporate atroom temperature. After removal from the casting surface, there isobtained a clear, self-supporting, tough film of DL-alaninehomopolyamide. A portion of this film is washed continuously with hottap water for 30 minutes and then dried at C. During this water wash,the DL-alanine homopolyamide film decreases approximately 27% in weightand increases in length by approximately 35%. Upon examination, a sampleof the water-washed film is found to exhibit mnh.=0.52 and 0.49 atconcentrations of 0.1 to 0.3%, respectively. 7

EXAMPLE IX A solution of 54 parts of DL-alanine N-carboanhydride[prepared as described previously in Example I and purilied by onerecrystallization as described previously in Example II] in 250 parts ofanhydrous diethyl ether is filtered and diluted with 300 parts ofreagent grade toluene in a glass reactor fitted with a reflux condenser.[The water adsorbed on the inner surfaces of this polymerization reactorwas previously removed by adding about 35 parts of anhydrous diethylether and distilling this to the atmosphere] The ether/toluene solutionof the N-carboanhydride is heated with stirring, protected from theatmosphere by a drying tube. The diethyl ether is removed bydistillation and the resulting toluene solu- 1 tion of theN-carboanhydride heated at the reflux with stirring for 46 hours. Theproduct, which separates out during this time, is removed from thepolymerization mixture by filtration, vacuum dried and taken up informic acid. The resultant viscous solution is flowed in a thin filmonto a glass plate and the formic acid allowed After removal from thecasting surface, there is obtained a clear, strong, tough film of a highmolecular weight DL-alanine homopolyamide. Samples of this film exhibitinherent viscosities of 0.36 and 0.31 at 0.1 and 0.3% concentrations,respectively.

Another sample of DL-alanine N-carboanhydride, similarly prepared andrecrystallized, upon being polymerized under anhydrous conditions inreagent grade toluene at a solids concentration of approximately 3.8% atthe reflux under anhydrous conditions for approximately 98 hours yieldsa DL-alanine homopolyamide exhibiting inherent viscosities of 0.46 and0.40 at concentrations of 0.3 and 0.5%, respectively, in formic acid.

While other methods are well known in the art for the preparation ofalpha-amino acid N-carboanhydrides, e. g., those of Leuchs, Ber. 39, 857(1906), Leuchs and Geiger, Ber. 41, 1721 (1908) and Bergmann et al.,Ber. 65, 1192 (1932) and J. Biol. Chem. 111, 245 (1933) whereinalpha-amino acid N-carboanhydrides are prepared by a Series of reactionsinvolving the conversion of the amino acid or its ester to thecorresponding N-carbomethoxy, carboethoxy, or carbobenzyloxy derivativesand the treatment of this intermediate derivative with thionyl chlorideorphosphorus pentachloride to form the corresponding N-carboalkoxy acidchloride and finally the formation of the desired N-carboanhydride fromthis intermediate through a ring closing reaction involving liberationof alkyl chloride and the much less convenient method of Curtiusinvolving the heat induced rearrangement of malonic azide acids andesters (see for instance, Curtius, J. prakt. Chem., 125, 211 (1930),Curtius and Sieber, Ber. 55, 1543 (1922), and Wesseley, Z. physiol.Chem. 146, 72 (1925)), nevertheless the preparative methods of MacDonaldand Prichard, which are discussed in detail in the previously identifiedcopending applications of these inventors and which involve the directreaction of phosgene with the necessary amino acid or salt thereof, aregreatly superior to any of the above discussed literature methods notonly in convenience, cost, and availability of reactants, but also to anoutstanding degree in reaction efliciency as judged by yields andconversions. Furthermore, for reasons as yet unknown, it has thus farproven impossible to prepare the oriented, high molecular weightDL-alanine homopolyamides of this invention from DL- alanineN-carboanhydride prepared by any other method than those of MacDonaldand Prichard, no matter under what conditions the polymerization iscarried out nor how many times the N-carboanhydride is recrystallized.It thus appears at present that, particularly as to impurities and sideproducts present and, therefore, as to proper methods of purification,that DL-alanine N-carboanhydride of the levels of purity necessary forthe preparation of the oriented, high molecular weight DL-alaninehomopolyamides of this invention can only be prepared by the directreaction of phosgene on DL-alanine or its salts by the methods ofMacDonald and Prichard.

For the preparationof the oriented high molecular weight DL-alaninehomopolyamides of this invention, it is necessary that the DL-alanineN-carboanhydride however prepared be in a high state of purity. In thoseinstances where purification proves necessary, a convenient and readymethod for carrying out this purification to the necessary levelsinvolves the recrystallization of the N-carboanhydride by aprecipitation-crystallization technique from anhydrous diethyl ether andpetroleum ether mixtures. Of course, other methods of purification knownin the art may be used, e. g., chromatography, sublimation, anddistillation (especially high vacuum, essentially molecular). However,the above-mentioned precipnation-crystallization technique is preferredbecause of its simplicity and efiiciency.

For example, in recrystallizing parts of DL-alanine N-carboanhydride, itis necessary merely to dissolve the N-carboanhydride in about 80 partsof warm anhydrous diethyl ether (i. e., at temperatures fr-om 30 to 35C., but below the boiling point) and to remove any undissolved materialfrom the resulting solution by filtration under anhydrous conditions.Petroleum ether is then added slowly with stirring to the resultingclear solution until the point of first noticeable and persistentcloudiness is reached (usually this requires about 15 to 30 parts ofpetroleum ether). The resulting cloudy solution is then cooled to about0 C. The DL-alanine N-carboanhydride begins to crystallize out in arelatively short time. When no apparent further crystallization occurs,additional petroleum ether is added until a total amount correspondingto approximately one to two or more times the volume of the diethylether used has been added. The resulting mixture is allowed to stand atabout 0 C. for approximately an additional 20 to 30 minutes, and thepurified crystalline DL-alanine N-carboanhydride removed by suctionfiltration and dried on the filterall under anhydrous conditions.

The recrystallization may, of course, be carried out at temperaturesboth lower and higher than 0 C. However, if temperatures appreciablylower than 0 C. are used, the purification effected is nowhere near ascomplete. If temperatures appreciably higher than 0 C. are

used, the purification effected gives no evidence of being noticeablybetter. The increased time necessary and the lowered efficiency ofrecovery both operate against such conditions being those of preference.

Adequate criteria of purity of the DL-alanine N-carboanhydride necessaryto prepare the high molecular weight oriented DL-alanine homopolyamideof this invention are that the N-carboanhydride (a) contain no chlorineWithin the limit of error of the analytical method used (generallymicroor semirnicrogravimetric procedures); '(b) contain 12.2% :0.2%nitrogen (12.2% being the calculated nitrogen percentage) as determinedby the Dumas procedure; and (c) exhibit a specific ultravioletextinction of maximum value in the neighborhood of 2550 A., falling offrapidly until a value of less than 0.0015 is reached in the vicinity of2700 A. with no increase above 0.0015 and a general gradual decrease ofthe specific extinction in the range up to and including 3200 A.

The melting point of DL-alanine N-carboanhydride is, in general, of noparticular value in indicating the relative purity of the materialbecause of the fact that, being a temperature of melting withdecomposition, it varies over a wide range, depending on the rate ofheating and the temperature at which the determination is started. Thesefactors are, of course, relatively difficult to control for a compoundmelting in the relatively low temperature range that DL-alanineN-carboanhydride does (i. e., 45- 55 C.).

The three criteria of purity mentioned above are sufiicient inthemselves for determining whether a given sample of DL-alanineN-carboanhydride is capable of polymerization under the previouslydiscussed preferred conditions to a high molecular weight, orientableDL-analine homopolyamide of this invention. It has been found that DL-alanine N-carboanhydride which has been recrystallized two to threetimes by the above-previously-described recrystallization technique, iscapable of forming the desired polyamide. Accordingly, rather thanobtain the time consuming data discussed above for each sample of theN-carboanhydride, it has been found most expedient to recrystallize theDL-alanine N-carboanhydride at least two, and usually three, times priorto polymerization.

In the previously mentioned copending applications of MacDonald andPrichard, it has been pointed out that the alkali metal, alkaline earthmetal, and hydrohalide salts of alpha-amino acids as Well as the acidsthemselves can be used in the preparation of the correspondingN-carboanhydrides by reaction with phosgene. In prepaiing the startingDL-alanine N-carboanhydride necessary for obtaining the products of thisinvention, it is preferred to use the free acid itself, i. e.,DL-alanine, since it is more readily available at relatively low costs.Adequate criteria of purity of the starting DL-alanine are that itexhibit upon analysis: a neutralization equivalent (N. E.) of 89110.45(the calculated value being 89.1), only traces 1 1 of inorganiccontaminants after ashing, and no ultraviolet absorption above 2500 A.Although many of the commercially available grades of DL-alanine aresufficiently pure as received, some others have been found to beinsufficiently pure. A ready method of purifying such materials to thedesired level is to recrystallize the DL-alanine from approximately 3parts of distilled water per part ot amino acid by heating the sample upto just below the boiling point with the correct amount of water,filtering the hot solution, and allowing it to cool and finally to standovernight at room temperature.

For the best yields of DL-alanine N-carboanhydride of the highestquality, thorough precautions should be taken to make sure that thestarting DL-alanine is as free of adsorbed water vapor as possible.This, of course, becomes particularly pertinent during hot and humidweather conditions since it is normally impractical to store thestarting DL-alanine in as carefully conditioned an atmosphere as wouldbe desired. It has been found that baking the, DL-alanine at 95 to 105C. for a period of approximately 16 to 18 hours immediately beforeconversion to the N-carboanhydride appreciably increases the yield ofgood quality N-carboanhydride obtained. This baking has also been foundto increase at least slightly the yield of N-carboanhydride even whenthe reaction is carried out under generally rather dry atmosphericconditions. This latter improvement is presumably due to the high andrelatively long term water adsorptability of DL -alanine as normallyobtained (a very fine powder with a relatively high surface to weightratio).

Many methods of polymerization of N-carboanhydrides to high molecularweight polyamides are known in the art, e. g., bulk fusion,polymerization in suspension in unreactive liquid organic media, etc.However, as pointed out previously in the specification, for thepreparation of DL-alanine homopolyamide of sufiiciently high molecularweight (e. g., inherent viscosity of at least 0.3) to allow thepreparation of the tough, strong, water sensitive, oriented films andfibers of this invention, it is necessary that the polymerization of theN-carboanhydride be carried out in an organic liquid nonreactive withthe DL-alanine N-carboanhydride at a concentration no greater than about14% and preferably in the range of 0.5 to 10.0%. Polymerizations carriedout at solids concentrations in the lower portions of this range,usually from 0.5 to 3.0%, generally yield higher molecular weightpolymer in less time. Furthermore, polymerizations carried out in theselower concentrations generally lead to the preparation of the highestmolecular weight polymer. Examples of such organic liquids includehalogenated hydrocarbon solvents such as chloroform, carbontetrachloride, tetrachloroethane, dichloroethylene, chlorobenzene, andtetrachloroethylene; liquid aromatic hydrocarbons, such as the xylenes,toluene, etc. However, as mentioned previously in the specification, thebest results, i. e., the most desirable DL-alanine homopolyamides, areobtained when the polymerization is carried out in benzene solution.

As has been previously mentioned in Example III, the

molecular weight of the DL-alanine homopolyamides, as evidenced byviscosity measurements, markedly increases as the concentration of theDL-alanine N-carboanhydride decreases with respect to the organic,liquid polymerization medium being used. Thus, other conditions beingthe same, such as the degree of purity of the DL-alanineN-carboanhydride and the temperature and time of polymerization, highermolecular Weight DL- alanine homopolyamide is obtained by polymerizingat lower concentrations. Such an increase becomes appreciably morenoticeable as the concentration is decreased to the range of 1% or less,based on the organic, liquid, polymerization medium.

For the preparation of the highest quality polymer, as has already beenpointed out, it is desirable to use reagent grade benzene as thepolymerization medium. t is also beneficial to distill a small portionof the benzene'medium from the polymerization reactor at the start ofthe polymerization. It has. been found that such a procedure serves toremove the traces of water usually adsorbed on the inner surface of theglass equipment normally used. For convenience, it is normally preferredto prepare an anhydrous diethyl ether solution of the DL-alanineN-carboanhydride, mix this with enough reagent grade benzene to give thedesired solids concentration. in the polymerization reactor to be used,and then to heat the polymerization reactor to the desired temperature,removing the diethyl ether and a small portion of the benzene bydistillation and finally maintaining the reaction mixture under refluxat the desired temperature for the desired time, The quantities of waterremoved as the well-known benzene/ water binary by this method areinmost instances relatively minor. However, even these minor quantitiesof appreciable magnitude in comparison with the known concentration ofwater involved in the reagent benzene used as the polymerization'medium(i. e., less than 0.02%).

'iemperatures in the range of 40 C. or as high as 150-200 C. may be usedin the polymerization cycle with superatmospheric pressure whennecessary. However, to insure the preparation of the best quality DL-alanine homopolyamide in a reasonable amount of time, temperatures of atleast 65 C. and preferably in the neighborhood of C. have been found tobe preferred. Although polymerization is much more rapid at highertemperatures, to insure the preparation of the highest qualityDL-alanine homopolyamides, it is preferred to use temperatures no higherthan C. and preferably no higher than in the neighborhood of 80 C.Similarly, although superatmospheric pressures may be used, the bestquality DL-alanine homopolyamide can most readily be prepared bypolymerizing under the obviously more convenient atmospheric pressure.

Although polymerization times of as short as 20 hours may be used, it ispreferred to carry out the polymerizations for from 36 to 72 hours attemperatures in the neighhood of 80 C. Obviously, there is no effectiveupper limit to the times of polymerization since the polymerization canbe carried out for as long as is desired. However, in the interests ofpracticability, it has been found that carrying out the polymerizationfor longer than 168 hours shows no improvement in the molecular weightof the DL-alanine homopolyamides produced.

No specific illustration is given in the foregoing examples of any addedpolymerization initiators other than the relatively minute quantities ofwater in the reagent grade benzene polymerization medium, i. e., lessthan 0.02%. However, it is to be understood that minute quantities ofother initiators may be added to the polymerization system if desired;Suitableinitiators are disclosed in the copending applications ofMacDonald Serial No. 778,031 and Serial No. 778,032, filed October 4,1947.

Of these added initiators, it is preferred to use theaminohydrogen-containing amines since they serve more effectively toboth initiate and control the polymerization, and it is relatively moreeasy to obtain high molecular weight polyamides therewith. I

The foregoing detailed description has been given for clear'ness ofunderstanding only and no unnecessary limi tations are to be understoodtherefrom. The invention is not limited to the exact details shown anddescribed for obvious modifications will occur to those skilled in theart.

What is claimed is:

1. Process of preparingan oriented, high molecular weight, linearpolyamide, the recurring units of which are DL-alanine units whichcomprises heating DL-alanine= N-carboanhydride, at 55415 C. for at least20 hours in solution in an inert solvent, liquid at the reaction ternper ature, for the said DL-alanine-N-carboanhydride but which is anon-solvent for poly-D'L-alanine, said solution containing not more thanfourteen per cent, by weight, of DL-alanine N-carboanhydride, saidN-carboanhydride being analytically free from chlorine, containing 12.2%:0.2% nitrogen as determined by the Dumas method, and having a specificultraviolet extinction of maximum value about 2550 A. falling offrapidly to a value below 0.0015 in the vicinity of 2700 A., with noincrease above 0.0015 anda general gradual decrease in specificextinction in the range to and including 3200 A., continuing thepolymerization until the resulting polyamide has an inherent viscosityof at least 0.3 and orienting the polyamide by stretching the polyamide,swollen with a swelling agent therefor, until the stretched length ofthe polyamide is at least 200% of its unstretched length.

2. Process of preparing an oriented, high molecular weight, linearpolyamide, the recurring units of which are DL-alanine units whichcomprises heating DLalanineN- carboanhydride, at 55-115 C. for at least20 hours in solution in an inert solvent, liquid at the reactiontemperature, for the said DL-alanine-N-carboanhydride but which is anon-solvent for poly-DL-alanine, said-solution containing not more thanfourteen per cent, by weight, of DL-alanine N-carboanhydride, saidN-carboanhydride being analytically free from chlorine, containing 12.2%i0.2% nitrogen as determined by the Dumas method, and having a specificultraviolet extinction of maximum value about 2550 A. falling of!rapidly to a value References Cited in the file of this patent UNITEDSTATES PATENTS 2,071,253 Carothers Feb. 16, 1937 2,516,145 Prichard July25, 1950 2,534,283 MacDonald Dec. 19, 1950 2,540,855 Tullock Feb. 6,1951 OTHER REFERENCES Go et al.: Bull Chem. Soc. Japan, 1939, pages 510,5l2, 514, 516.

Frankel et al.: Journ. Am. Chem. Soc.., vol. 64, 1942, pp. 2264 to 2271.

Ofiice of Technical Services, PB 34,279, Dec. 13, 1946, 3 pp.

Astbury et al.: Nature, vol. 162, Oct. 16, 1948, page 596.

Alfrey: Mechanical Behavior of High Polymers, Interscience, 1948, page 50O.

1. PROCESS OF PREPARING AND ORIENTED, HIGH MOLECULAR WEIGHT, LINEARPOLYAMIDE, THE RECURRING UNITS OF WHICH ARE DL-ALANINE UNITS WHICHCOMPRISES HEATING DL-ALANINEN-CARBOANHYDRIDE, AT 55-115* C. FOR AT LEAST20 HOURS IN SOLUTION IN AN INERT SOLVENT, LIQUID AT THE REACTIONTEMPERATURE, FOR THE SAID DL-ALANINE-N-CARBOANHYDRIDE BUT WHICH IS ANON-SOLVENT FOR POLY-DL-ALANINE, SAID SOLUTION CONTAINING NOT MORE THANFOURTEEN PER CENT, BY WEIGHT, OF DL-ALANINE N-CARBOANHYDRIDE, SAIDN-CARBOANHYDRIDE BEING ANALYTICALLY FREE FROM CHLORINE, CONTAINING12.2%+0.2% NITROGEN AS DETERMINED BY THE DUMAS METHOD, AND HAVING ASPECIFIC ULTRAVIOLET EXTINCTION OF MAXIMUM VALUE ABOUT 2550 A. FALLINGOFF RAPIDLY TO A VALUE BELOW 0.0015 IN THE VICINITY OF 2700 A., WITH NOINCREASE ABOVE 0.0015 AND A GENERAL GRADUAL DECREASE IN SPECIFICEXTINCTION IN THE RANGE TO AND INCLUDING 3200 A., CONTINUING THEPOLYMERIZATION UNTIL THE RESULTING POLYAMIDE HAS AN INHERENT VISCOSITYOF AT LEAST 0.3 AND ORIENTING THE POLYAMIDE BY STRETCHING THE POLYAMIDE,SWOLLEN WITH A SWELLING AGENT THEREFOR, UNTIL THE STRETCHED LENGTH OFTHE POLYAMIDE IS AT LEAST 200% OF ITS UNSTRETCHED LENGTH.